The present invention relates to an incremental optical encoder.
The principles of optical incremental encoders are well known. In summary, a thin disc is provided with a multiplicity of slits around its peripheral edge. The number of slits and their spacing determines the degree of control which can be exercised on the motor upon which the encoder is mounted.
On one axial side of the disc is mounted a light emitting diode which is made to send a beam of light perpendicular to the face of the disc. When this light beam is opposite a slit in the disc the light passes through to land on a light sensitive sensor which changes logical state from zero to one. As the disc rotates so the slit moves on and the light beam is cut off by the disc material between the slits. The light sensor returns to logic level zero.
It is common to provide two light emitting diodes and two light sensitive elements each side by side on opposite sides of the disc. By precise placing of the two sets of electronic devices they can be made to give up/down signals 90 degrees out of phase with one another. Because of this it is possible to use the two signal channels to determine the direction of rotation of the motor. This is essential for full servo control. Since both the up edges and the down edges of each signal can be used then there are four signals for each disc slit. This further enhances servo control.
One problem faced by designers of these encoders is concerned with the fact that the light emitting diodes propogate a dispersant beam. Some degree of masking can reduce this. However the fact that the disc can move axially between the emitters and receivers can cause a dispersed beam to reach a sensor. The consequence of this is that the time when the sensor is at logic level zero and the time at logic level one are not equal. This is often called the signal duty cycle. If these duty cycles are allowed to vary substantially then the states of both sensors cannot be compared to sense direction any more since one of the signals, having a short up time may go down before the other sensor which may have a long up time hence spoiling the sequence.
An incremental optical encoder has been proposed in U.S. Pat. No. 3,400,275 which has a light source that is movable with respect to a projection means which projects light from the light source towards a disc, for adjusting the ratio of light intensities or the collimation of the light. However, this proposal has the disadvantages that the rigidity of mounting of the light source is compromised and the manufacture of the movable light source does not lend itself to easy automation, thus leading to increased expense.